The construction cost of floors and roofs is directly affected by the amount of material used and the depth of floor (roof) construction. Depth of floor construction is referred to as the zone enclosed by the floor surface on the top and the ceiling at the bottom. This zone contains the girders, beams and slabs required for the structure; and the air conditioning ducts, plumbing and lighting elements, etc., required for the mechanical sub-systems. The greater this depth is, the taller the building must be for the same height of useful living space. In order to achieve savings in the weight of steel beams and the depth of floor construction, "composite construction" has been increasingly utilized.
Several composite construction systems are available. The basic and most commonly used composite construction, combines a reinforced concrete slab and an "I" shaped steel beam in a single load carrying component, in which the concrete slab resists the compression and the steel beam the tension component produced by the bending moment. In this mode of construction, shear connectors are welded to the top flange of the steel beam. These connectors, when embedded in the concrete slab, ensure that the two materials work in unison. Shear connectors may be of several types. See U.S. Pat. No. 2,987,855 and U.S. Pat. No. 3,210,900. Because the thickness of the concrete slab is usually controlled by considerations other than the compression due to composite action, there is a large excess capacity available for the resistance of these forces. The design of the steel beam is governed by the tension in the bottom flange. As a consequence, when a rolled "I" beam is used for composite construction, because its top and bottom flanges are identical, its top flange is not fully utilized. This lack of full effectiveness of the top flange in composite construction was reported on by A. A. Toprac in AISC Engineering Journal, "Strength of Three New Types of Composite Beams", January 1965.